1. Field of the Invention
The present invention relates to an optical filter for preventing the introduction of spurious color signals in a video system and more particularly to a double layered optical low pass filter capable of providing an optimum cut off spatial frequency for the primary colors to prevent spurious color signals while passing higher spatial frequency luminance signals to permit an improved image resolution.
2. Description of the Prior Art
In the field of color television camera systems, the use of a color encoding or dichroic filter for modulating the light flux and/or optically converting an object scene into high frequency signals is well known. In color television the transmission of a color picture representative of the object scene requires three independent video signals and various forms of single and double tube camera systems have been utilized such as U.S. Pat. No. 2,733,291 and U.S. Pat. No. 3,378,633.
Various forms of dichroic or color encoding filters have been utilized in the prior art, for example, U.S. Pat. No. 3,771,857 and U.S. Pat. No. 3,860,955. These color encoding filters are placed in the light path of a pickup tube or image tube to separate the light passing through it into primary color component light signals. These light signals are then transformed into an electrical signal after they impinge upon a photosensitive element of the pickup tube. The image plane of the optical system is focussed on the pickup tube and an electron beam scans a raster in deriving the electrical signals.
In addition to the primary color signals generated, the color encoding filters also provide, throughout their grid, areas that are transparent to the primary colors and thus pass a light representative of the brightness of the image. The color signal components and luminance signals can then be electrically separated by circuitry external to the pickup or image tube. The separate signals are then processed in a manner to produce wave forms for direct application to a color receiver or to produce a composite wave form conforming to broadcasting standards for application to a transmitter.
A problem that is recognized in the prior art, is the interference or cross talk between high frequency luminance signals and the chrominance signals. If the object scene contains high spatial frequency components which fall into the chrominance signal band, spurious signals are produced by the interference between the luminance and chrominance signals. These spurious signals can originate for example from stripe patterns in the object scene or from edges in the object scene since the Fourier decomposition of an edge has frequencies in the appropriate range. The decoding scheme in the video system will erroneously interpret these spurious higher spatial frequencies as color information and accordingly incorrect colors will be observed in the reconstruction of the object scene. In addition, strong moire patterns have been observed which are created between the interaction of the color encoding gratings with the gratings produced by the appropriate range of spatial frequencies in the object scene.
Thus, it has been known in the prior art that it is highly desirable to eliminate any beat frequencies and attempts have been made to optically defocuss the optical image formed at the target electrode of the image tube. However, it is desired that the luminance representative signal should have as high a resolution as possible, in order to reproduce the object scene with sufficient detail. By simply optically defocussing the optical image there would be a reduction in the luminance resolution.
One approach to this problem, has been to insert an astigmatic filter having alternate and parallel strips of different transmissivity and having a spatial cut off frequency around the carrier frequency of the lowest frequency color component carrier signal derived from the image tube, see U.S. Pat. No. 3,566,013.
Another approach has been to use a series of birefringement elements that are rotationally mounted within the optical axis of the video system, such as disclosed in U.S. Pat. No. 3,588,224.
Another solution has been to use a symmetrical rectangular wave phase grating so that those spatial frequencies in the object scene which produce a beat interference are filtered out and the problems of color misinformation and moire patterns are eliminated, an example of this type of optical filter can be seen in U.S. Pat. No. 3,681,519.
The use of rectangular wave phase grating which includes a plurality of sets of laminae to attenuate striped diffraction patterns of Fresnel order of a defocussed image and color striped patterns affected by the interference between the color encoding filter and the striped diffraction patterns is disclosed in U.S. Pat. No. 3,768,888 owned by the assignee of the present invention.
An optimized optical low pass filter utilizing phase grating to attain a response to zero in a frequency over a desired cut off frequency while at the same time being independent of the F number of the optical system is disclosed for respectively a rectangular phase grating and a trapezoidal phase grating in U.S. Pat. Nos. 3,756,695 and 3,821,795 also owned by the assignee of the present invention.
Recently, an optical filter which is formed from several randomly distributed parallel grating stripes which overlap each other has been disclosed in U.S. Pat. No. 3,911,479. For simplification, this form of grating with random overlapping distribution is described as a Poisson grating. Purportedly, this filter is capable of providing an integer multiple of phase differences in the green spectrum, as a result of the parallel distribution of the respective grating stripes deposited on the substrate. Each of the respective layers of stripes are aligned parallel to the other stripes and may be either positioned directly on a substrate or actually deposited to physically overlap a previously deposited stripe. The production problems associated with this type of an optical filter can be considerable, since when the grating stripes are overlapped, misalignment can easily occur with a resulting effect upon the phase retardation and grating strip distribution. As can be readily appreciated, the utilization of an optical low pass filter in the television industry may be cost competitive. In the case of a regularly spaced phase retardation grating, the grating space is in the order of 1 mm to 100 .mu. and it becomes almost impossible to economically fix the gratings at the desired thickness for the phase retardation stripes by any form of multiple evaporation process.
The desire to provide an economical optical low pass filter that can eliminate spurious signals produced by the interference between luminance and chrominance signals while permitting an improved resolution is still a goal of the prior art.